Color theory encompasses a multitude of definitions, concepts and design applications - enough to fill several encyclopedias. However, there are three basic categories of color theory that are logical and useful : The color wheel, color harmony, and the context of how colors are used.
The Color Wheel
Color Harmony - Color harmony is when you use color combinations side-by-side that are pleasing to the human eye. This can either create contrast or consonance (when you use similar shades)—so long as they make sense together and create a visually satisfying effect.
Color Context - Color context, or the makeup of colors that are next to each other, make a dramatic difference in how color is perceived.